A variety of imaging techniques are currently being used in the medical field. For examples, positron emission tomography, single photon emission computed tomography, and computed tomography, are some of the examples of the imaging techniques that are commonly used.
Positron emission tomography (PET) detects photons generated through positron-electron annihilation of positrons from a radioactive tracer placed in the object, e.g., patient, to be imaged, and analyzes the photon energy and trajectory to generate tomographic images of the patient. Single photon emission computed tomography (SPECT) generates images by computer analysis of photon emission events from a radioactive tracer. Positron-electron annihilation may be the source of such photon emission. PET and SPECT require the detection and analysis of single photon events. Photomultipliers are generally used for single photon event detection in PET and SPECT.
Computed tomography is an imaging technique that has been widely used in the medical field. In a procedure for computed tomography, an x-ray source and a detector apparatus are positioned on opposite sides of a portion of a patient under examination. The x-ray source generates and directs a x-ray beam towards the patient, while the detector apparatus measures the x-ray absorption at a plurality of transmission paths defined by the x-ray beam during the process. The detector apparatus produces a voltage proportional to the intensity of incident x-rays, and the voltage is read and digitized for subsequent processing in a computer. By taking thousands of readings from multiple angles around the patient, relatively massive amounts of data are thus accumulated. The accumulated data are then analyzed and processed for reconstruction of a matrix (visual or otherwise), which constitutes a depiction of a density function of the bodily section being examined. By considering one or more of such sections, a skilled diagnostician can often diagnose various bodily ailments such as tumors, blood clots, etc.
Some radiation therapy linacs are equipped with a diagnostic quality x-ray imaging system. The flat panel x-ray detector and KV x-ray source are attached to the linac gantry and rotate with the gantry around the patient. This imaging system is used to locate and target tumors and sensitive organs around the tumor inside the body. This increases the accuracy of targeting the linac treatment beam at the target, while avoiding the surrounding healthy tissue. One method of 3D imaging using such systems is cone beam CT imaging where of 2D x-ray projections acquired as the gantry rotates around the patient. Using cone beam CT reconstruction, a 3D image of patient anatomy is formed.
PET, SPECT, and Computed tomography has found its principal application to examination of bodily structures or the like which are in a relatively stationary condition. However, currently available PET, SPECT, and computed tomographic apparatus may not be able to generate images with sufficient quality or accuracy due to physiological movement of a patient. For example, beating of a human heart and breathing have been known to cause degradation of quality in PET, SPECT, and CT images.
Degradation of quality of images due to patient's breathing is more difficult to address than that associated with heart motion. Patients' breathing poses a unique problem to imaging that is different from heart motion. This is because the pattern and the period of a patient's breathing cycle is generally less consistent when compared to those of the patient's cardiac cycle. As such, while a particular phase of a cardiac cycle may be predicted with sufficient accuracy, a particular phase of a breathing cycle may not be as easily predicted or determined.
Currently, to account for a patient's breathing during an image acquisition session, the patient's breathing motion is measured and recorded while image data are obtained. After all of the image data are collected, the breathing motion data are analyzed to determine phases of a respiratory cycle. The determined phases are then associated with the image data (e.g., PET, SPECT, or CT image data) such that image data generated at the same phase of the respiratory cycle are associated with the same phase. In existing systems, all of the image data are obtained before they are subsequently associated with the motion data. However, the wait time for finishing the image acquisition process can be quite long, especially for the case of PET imaging. In PET imaging, the list mode of image data acquisition may take up to 45 minutes. As a result, image data that are associated with motion data (e.g., binned image data) may not be available to a physician until a long time (e.g., sometimes in the order of minutes) after the image data is acquired.
In the case of respiratory correlated, or 4D, cone beam CT, the acquired 2D projections are associated with a discrete and finite set of periodic breathing phase intervals called phase bins. A separate 3D image is formed by reconstructing from the projections corresponding to each phase bin. The sequence of 3D images corresponding to the designated phase bins are called 4D cone beam CT images. In order to have a more uniform distribution of projections corresponding to a bin on the gantry rotation circle, the gantry rotation speed is reduced such that acquisition of projections can take several minutes. Again, waiting for the completion of acquisition of the projection images before reconstruction of images begins can take a very long time.
For the foregoing, improved system and method for processing image data, and more particularly, for associating physiological data with image data, would be desirable.